Interpretive Summary: High milk yield early in lactation affects breeding decisions and lowers conception rates; early conception reduces yield late in lactation. To partition and to measure the effect of pregnancy on yield so that environmental and genetic factors can be distinguished and accounted for appropriately, milk, fat, and protein yields early in lactation, current days not pregnant (days open), and 305-day yields were investigated for Holstein cows. Longer days open was extremely beneficial to high 305-day yield as days open increased to 100 days; after 100 days open, the gain was small. Impact of current days open was greater for second lactation than for first. If early yield to 120 days was considered, the difference was reduced. The reduced estimates of the effect of current days open on lactation yield show the importance of accounting for early lactation yield in developing factors to standardize lactation records that are used in genetic evaluations. Too much adjustment had been advocated in previous studies because the statistical models did not consider early lactation yield. New adjustment factors for days open based on results of this study will improve standardization of lactation records and increase accuracy of genetic evaluations used by dairy producers for breeding decisions.

Technical Abstract:
To partition and measure the effect of pregnancy on yield, relations among milk, fat, and protein yields early in lactation, current days open, and 305-day yields were investigated with sample-day records of 247,310 Holstein cows. The model included fixed effects of calving herd-year-season, calving age, and days open, and a random residual effect. Early cumulative yield to 80, 100, 120, or 140 days was included in the model as a continuous variable to address interaction between high yield and delayed breeding. Longer days open was extremely beneficial to high 305-day yield as days open increased to 100 days; after 100 days open, the gain was small. Impact of current days open was greater for second lactation than for first; difference in 305-day milk yield between cows open 40 and 290 days was 1199 kg for first lactation and 1613 kg for second lactation. If early yield to 120 days was included in the model, the difference was reduced to 860 kg for first lactation and 1001 kg for second lactation. Inclusion of early yield in the model reduced regression coefficients for days open during first lactation by 22% for 80-day yield, 24% for 100-day yield, 27% for 120-day yield, and 30% for 140-day yield and by 31%, 35%, 38%, and 41%, respectively, for second lactation; reductions were similar for fat and protein yields.